Optical interferometer gyroscopes can sense rotation by measuring the difference in time it takes for light or other eletromagnetic waves to pass in opposite directions through a common path loop whose rotation is to be measured. The light beam moving in the direction of rotation will arrive at the detectors later than the light beam moving opposite to the direction of rotation around the loop, and therefore the two light beams will be out of phase. If the two beams are only slightly out of phase, then the light intensity at the detector will not vary significantly for small changes in phase difference between the two beams, because the light intensity vs. phase difference curve will be at a location of substantially zero slope. As described in our earlier patent application, however, sensitivity can be greatly increased by shifting the phase of one of the beams by 90.degree., so that the detector is operating at a point on the light intensity vs. phase difference curve, which is of maximum slope, and therefore small variations in phase difference will result in large variations in light intensity.
Many practical difficulties hamper the development of a practical optical interferometer gyroscope system. One difficulty is in the generation of a 90.degree. phase shaft between waves travelling in opposite directions. Such a shift can be achieved by the use of a nonreciprocal device such as a Faraday cell, which utilizes a magnetic field to advance the phase of light travelling in one direction and delay the phase of light travelling in the opposite direction, but such devices have been found to be cumbersome and difficult to embody in single mode waveguide form. Another difficulty is that identical non-circular polarization states must be maintained throughout the length of the waveguide in order to obtain stable outputs. Otherwise, minor fluctuations in temperature, acceleration, stress, electric or magnetic field gradients, or even long term fiber relaxation will introduce large random phase changes at the outputs. Still another difficulty is that, while a 90.degree. offset provides high sensitivity for small rotation rates or rates for which the rotation induced phase offset approaches an integer multiple of 180.degree., it is not desirable at rates where the rotation adds a phase difference close to an odd multiple of 90.degree., because the system then becomes insensitive and the readings become ambiguous. An optical interferometer gyroscope system which could be constructed compactly and which could provide high accuracy and definiteness throughout a wide range of rotation rates, would facilitate the construction of practical gyroscope systems.